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WASHINGTON — Behind the U.S. drive for harsh new sanctions on Iran is growing concern that the Iranians are acquiring the know-how to build a nuclear weapon. The latest worry is a purported Iranian secret document that lays out a plan for developing a critical component of an atomic bomb.

Several U.S. officials familiar with the reportedly secret Iranian technical document said that its authenticity has not been confirmed, but that it is part of a broader pattern of evidence suggesting Iran is laying groundwork to build a nuclear weapon. Iran has consistently insisted its nuclear program is for civilian purposes.

The officials spoke on condition of anonymity to discuss internal assessments. The document was first reported Sunday by the Times of London newspaper.

The U.S. and its European allies are growing impatient with what they see as Iranian intransigence over Obama administration and international efforts to ensure the regime is not on track to become a nuclear power. Decisions may come early next year to impose new penalties on Iran, which has defied three sets of U.N. Security Council sanctions aimed at forcing it to again freeze uranium enrichment.

White House press secretary Robert Gibbs said that an Iranian missile test Wednesday weakens Iran's claims that its nuclear program is intended for peaceful purposes, while solidifying support for new sanctions.

Missile tests 'not productive'
"I think there's no doubt that, given this environment, missile tests do nothing but undermine the Iranian claims," Gibbs told reporters. "They're not productive. The Iranians still have the opportunity to live up to their responsibilities. If they don't, then time will run out and we will move to the next step."

Gibbs said Russia and others disappointed by Iranian actions are now prepared to take new steps on sanctions.

Secretary of State Hillary Rodham Clinton capsuled U.S. frustration by telling reporters on Monday in blunt terms that the administration's efforts to engage Iran had "produced very little."

Against that background, U.S. intelligence agencies are weighing the significance of the reported Iranian technical document, which appears to describe a work plan for developing a neutron initiator, used to detonate a nuclear bomb.

A senior U.N. official who has seen the document said he could not yet make a determination on its authenticity. It was also unclear, the official indicated, when the document was written and whether it had remained simply as a blueprint for a program that was never carried out, or if actual experiments described in it had been performed.

But a senior western diplomat from one of the five permanent members of the U.N. Security Council said that the issue of when the document was written was of less significance than the concern that the document — if genuine — further strengthens assertions that Iran is interested in developing nuclear weapons.

Both officials spoke on condition of anonymity because they were not authorized to discuss intelligence matters.

Iran says documents fabricated
The Iranians have repeatedly asserted that Western intelligence agencies have fabricated documents in order to reinforce the case against Iran's nuclear program.

One of the U.S. government officials said the document had been known to American intelligence for more than a year and had already been factored into current analysis of Iran's nuclear program.

The document's contents, the official said, do not affect the fundamental conclusions of a 2007 U.S. national intelligence estimate that said Iran had suspended work on nuclear weapons development between 2003 and mid-2007.

The upshot of the 2007 national intelligence estimate is that Iran appears to want a nuclear weapon and is pursuing research and development projects that would allow it to build one if it makes the political decision to do so. So far, the U.S. believes the Iranian leadership has not crossed that threshold.

There is also the question of where the document came from.

Without alluding to the document, Israel's military intelligence chief said Tuesday that Iran is close to an unspecified "technological breakthrough" that would enable it to build nuclear weapons. Maj. Gen. Amos Yadlin did not elaborate on the breakthrough but said the countdown on Iran's nuclear technology clock "has almost finished ticking."

'Smoking gun'?
Israeli expert Ronen Bergman, an author who has written extensively on intelligence matters, told The AP on Wednesday: "I don't know if this document is authentic or not, or if Israel is involved or not" in the leaking of the document. "If true, then this is the smoking gun, because this (trigger) device has no other possible purpose other than for a warhead or nuclear device."

President Barack Obama has indicated he would reassess his approach to Iran at the end of this year. Expectations are growing that he will switch diplomatic tracks and announce a push for a broad international consensus to impose new sanctions aimed at crippling the Iranians' financially and further isolating them politically.

The administration is concerned about Iran's refusal to carry through on a tentative deal struck in October that called for Iran to ship the majority of its low-enriched uranium out of the country in exchange for fuel to run a research reactor.

The deal was seen by the U.S. and its negotiating partners as a step toward building confidence in Iran's claim that its nuclear program is designed entirely to generate power, not weapons.

Momentum toward sanctions
The administration also stepped up the momentum toward sanctions after the revelation in September that Iran was secretly building a second uranium-enrichment facility near the holy city of Qom.

The House voted Tuesday to impose new economic sanctions on Iran, but with no Senate action expected before next year, the vote was mainly a warning that the U.S. is ready to act on its own. The House sanctions would end access to U.S. markets for foreign companies selling refined petroleum products to Iran.

On Wednesday, Iran may have added to momentum for sanctions by test-firing an upgraded version of its most advanced missile, which is capable of hitting Israel and parts of Europe. British Prime Minister Gordon Brown said the test showed the need for tougher U.N. sanctions. U.S. intelligence officials, however, downplayed the significance of the test, noting that Iran has tested this missile several times in the past.

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